Conotruncal malformations are severe congenital cardiac defects that require surgery early in childhood. Infants born with these defects suffer from significant morbidity and mortality. Abnormal embryonic development of the outflow tract (a region defined as the outflow vessels including the junction with the myocardium, called the arterial pole) produces these defects. Neural crest ablation causes a wide variety of conotruncal malformations: direct effects include absence of outflow septation, and indirect effects are malalignment of the outflow tract. In recent studies of the neural crest ablation model in chick embryos, we have discovered a secondary heart field (SHF) in the ventral pharyngeal mesenchyme that provides cells that become cardiac and smooth muscle myocytes essential to the normal formation of the arterial pole. In the neural crest-ablation model, myocardium from SHF is not added properly to the growing arterial pole resulting in malalignment of the conotruncus in addition to failure of outflow septation. The malalignment defects are a component of two conotruncal malformations called double outlet right ventricle and tetralogy of Fallot. Our preliminary results indicate that these malformations are due to elevated FGF signaling after neural crest ablation based on four pieces of evidence from investigations performed in this laboratory: 1) FGF target genes are elevated after neural crest ablation; 2) Fgf8b, the most active isoform of FGF8 is elevated; 3) a reporter cell line for FGF registers elevated FGF8b signaling in the ventral pharynx; and 4) FGF8b antibody restores normal looping and rescues alignment of the outflow tract in neural crest-ablated embryos. These preliminary data support our overall hypothesis that normal development of the arterial pole depends on the regulation of FGF8 signaling in the pharynx by cardiac neural crest cells. We will test the specific hypotheses: that elevated FGF8b leads to abnormal arterial pole development by affecting proliferation, migration and/or differentiation of the myocardial component of the secondary heart field (aim 1); neural crest cells normally depress FGF8 signaling in the caudal pharynx by endocytosis of the FGF protein and/or by decreasing the transcription of the Fgf8b isoform (aim 2). In aim 1, we will expose explanted SHF to various concentrations of FGF8b and determine its effect on proliferation, migration, cell death and differentiation. We will electroporate an FGF8b expressing plasmid into the pharyngeal endoderm of chick embryos in ovo to correlate developmental events in the secondary heart field with outflow alignment. In aim 2, we will (truncated) [unreadable] [unreadable]